Specifications

Chapter 4 - Transient Protection of RS-422 and RS-485 Systems RS-422 And RS-485 Applications Ebook
Page 53 of 137 Manual Documentation Number: <DocNumber>
www.bb-elec.com/
www.bb-europe.com/
In some installations, there may be another source of unwanted energy
to consider. If there are high voltage cables running anywhere near the
data cables, the potential for a fault condition exists as a result of
insulation failures or inadvertent contact by an installer. This type of
surge could contact any number of conductors in the data cable,
presenting a "differential" surge to the data equipment. Although the
voltages and currents associated with this type surge are much lower
than the types of surges modeled by ANSI or IEC, they have a
particularly destructive quality of their own. Instead of dissipating
within several milliseconds, they can exist in a steady state condition
on the data network.
When Ground is not really Ground
Realizing that transient energy can be high frequency in nature leads to
some disturbing observations. At frequencies of this magnitude, it is
difficult to make a low impedance electrical connection between two
points due to the inductance of the path between them. Whether that
path is several feet of cable or thousands of feet of earth between
grounding systems, during a transient event there can be hundreds or
thousands of volts potential between different "grounds". We can no
longer assume that two points connected by a wire will be at the same
voltage potential. To the system designer this means that although RS-
422/485 uses 5V differential signaling, a remote node may see the 5V
signal superimposed on a transient of hundreds or thousands of volts
with respect to that node's local ground. It is more intuitive to refer to
what is commonly called "signal ground" as a "signal reference".
How do we connect system nodes knowing that these large potential
differences between grounds may exist?
The first step towards successful protection is to ensure that each
device in the system is referenced to only one ground, eliminating the
path through the device for surge currents searching for a return.